Doped and undoped silicon dioxides, which are commonly referred to as silicate glasses, are widely used as dielectrics in integrated circuits. Although silicon dioxide possesses a tetrahedral matrix which will impart a crystalline structure to the material under proper heating and cooling conditions, the silicon dioxides used as dielectrics in integrated circuits are typically amorphous materials. Chemical vapor deposition of silicate glasses has become of paramount importance in the manufacture of contemporary integrated circuits. For example, silicate glass doped with both boron and phosphorous is widely used as an inter-level dielectric and as a getter material for mobile sodium ions. As geometries are shrunk in the interest of faster operating speeds and lower manufacturing costs, capacitive coupling between narrowly-spaced conductive layers and structures within the circuits has become a significant performance-limiting problem. The increased capacitive coupling leads to RC delays and concomitant speed degradation in integrated circuits.
In order to reduce capacitive coupling within integrated circuitry, glasses having lower dielectric constants have been tried. Fluorine-doped glass is particularly interesting because the incorporation of fluorine into the tetrahedral silicon dioxide matrix significantly lowers the dielectric constant of the material. Fluorine-doped glass may be deposited by the plasma reaction of ethyl hexafluoride (C.sub.2 F.sub.6), methyl tetrafluoride (CF.sub.4), or nearly any fluorine-containing compound with tetraethylorthosilicate and diatomic oxygen gas (O.sub.2). Films deposited via this technique have proven to be unusable because they characteristically contain free fluorine atoms. Furthermore, moisture absorption by silicate glasses increases with increasing fluorine doping concentrations. Free fluorine atoms combine with the absorbed moisture to form hydrofluoric acid which gradually leaches out of the glass. As hydrofluoric acid rapidly attacks most metals, with aluminum being particularly vulnerable to attack, circuit degradation is assured. Over time, circuit unreliability, device degradation and/or failure can result.
It is a principal object of the present invention to provide an inter-metal dielectric structure having a low dielectric constant for reduced capacitive coupling between adjacent conductors which does not compromise the reliability of such structures.